1. Field of the Invention
The present invention relates to a polymer prepared by synthesizing a monomer having a derivative of cholic acid, deoxycholic acid or lithocholic acid bonded to norbornene, and then homopolymerizing the monomer, copolymerizing the monomer with maleic anhydride, or copolymerizing the monomer, maleic anhydride and 2-hydroxyethyl 5-norbornene-2-carboxylate and/or 5-norbornene-2-carboxylic acid, and its use as a photoresist.
2. Description of the Prior Art
As reported in U.S. Pat. Nos. 3,666,473, 4,115,128, and 4,173,470, the conventional photoresist is generally formed of a composition containing an alkali-soluble phenol- (or cresol)formaldehyde novolak resin and a substituted naphthoquinone diazide compound as a photosensitive substance. However, an integrated circuit increasingly has an enhanced integration degree, and a wavelength of the exposing machine used in a photographic lithography is, therefore, in the range of 200 nm to 300 nm which is a far ultraviolet region. A this far ultraviolet region, the photoresist containing the naphthoquinone as the photosensitive substance is too strong in light absorption, and low in sensitivity, such that it is difficult to use. As a result, it is necessary to develop a new photoresist capable of effectively being used at this far ultraviolet region.
The development of the new photoresist must be made such that various requirements for characteristics of the photoresist are satisfied, such as a high sensitivity, a contrast, a high resolution, and a dry etch resistance. Among these characteristics, the sensitivity is most important, and to increase the sensitivity, a concept of a chemical amplification was introduced. In this chemical amplification, activated species generated by a photochemical reaction serve as a catalyst, such that chemical reactions, such as a deprotection, and a cross-linking, can successively occur, whereby a total quantum yield of these reactions is high as compared with that of the initial catalyst production reaction.
Consequently, in order to attain the high sensitivity in a lithographic process for fabricating a semiconductor device, there are recently highly spotlighted chemical amplification type photoresists. Among them, polybisphenol protected with t-butoxycarbonyl group is reported as a resin with a greatest possibility, as described in U.S. Pat. Nos. 4,311,782, 4,405,708, and 4,491,628.
A study on a photoresist required for the development of a semiconductor chip of 1 G(giga) bit or more DRAM is actively progressive, now. Where an argon fluoride(ArF) excimer laser having a wavelength of 193 nm is used as an exposure source, polyvinylphenols according to the prior art can not be used, since they show a strong light absorption and a low sensitivity, due to aromatic rings included therein. As a result, the chemical amplification type photoresist with an alicyclic compound was proposed for the production of a photoresist fairly resistant to dry etch and transparent to argon fluoride excimer laser light of 193 nm wavelength. As for this photoresist, there was reported a structure, in which an aliphatic compound was bonded to a branched chain of an acrylic compound, or another structure, in which an aliphatic compound, as a dissolution inhibitor, is added to a polymer, as disclosed in U.S. Pat. Nos. 5,585,223, 5,691,111, and 5,756,850.